Circularly polarized gravitational waves from parity-violating scalar-tensor theory

Abstract: We study both primordial GWs and scalar-induced gravitational waves (SIGWs) in a class of the parity-violating scalar-tensor (PVST) theory, of which the Lagrangian is the linear combination of seven ghost-free parity-violating scalar-tensor monomials dubbed the ``Qi-Xiu'' Lagrangians. At linear order, we obtain the quadratic action for tensor perturbations and show that parity-violating terms associated with $\mathcal{L}_{1,2,5,6,7}$ render the tensor propagation polarization dependent, leading to chiral primordial spectra and a nonvanishing degree of circular polarization. At second order, we derive the EOM for SIGWs and identify the explicit parity-violating source terms. In particular, $\mathcal{L}_3$ and $\mathcal{L}_4$ enter exclusively through the source term for SIGWs, allowing parity violation to arise even when the linear GWs propagation remains effectively GR-like. During the radiation-dominated era, we compute the fractional energy density of SIGWs for both monochromatic and lognormal curvature power spectra. We find that, around the peak frequency, SIGWs in PVST gravity exhibit characteristic deviations from those in GR, resulting in a nonzero degree of circular polarization.